The European laid-open specification EP 0 903 770 A2 describes mercury-free halogen metal-vapor high-pressure discharge lamps, whose ionizable filling contains at least one volatile metal halide acting as a voltage-gradient former and further metal halides for producing light. The metal halides acting as voltage-gradient formers essentially perform the functions of the mercury in the mercury-free high-pressure discharge lamps. Halides of the metals Al, Bi, Hf, In, Mg, Sc, Sn, Tl, Zr, Zn, Sb or Ga are used as voltage-gradient formers. Halides of the metals Na, Pr, Nd, Ce, La, Dy, Ho, Tl, Sc, Hf, Zr or Tm primarily serve the purpose of producing light.
The European laid-open specification EP 0 883 160 A1 discloses mercury-free halogen metal-vapor high-pressure discharge lamps whose ionizable filling contains an inert gas, a first metal halide for producing light and a second metal halide which acts as the buffer gas and has a high vapor pressure. Halides of the metals iron, cobalt, chromium, zinc, nickel, manganese, aluminum, antimony, beryllium, rhenium, gallium, titanium, zirconium or hafnium can be used as the buffer gas.